1. Field of the Invention
This invention relates to a process for purifying an aqueous solution of acrylamide obtained by the catalytic hydration of acrylonitrile with water in the presence of a copper-based catalyst.
2. Description of the Prior Art
Acrylamide has been used as acrylamide polymers which are useful as papermaking chemicals, flocculants, oil recovery additives, soil hardeners, etc., and has also found wide applications as a comonomer for other polymers. Early production of acrylamide was by the so-called sulfuric acid process. Recently, a process involving catalytic hydration of acrylonitrile in the presence of a copper-based catalyst was developed, and has now superseded the sulfuric acid process in industrial production.
Flocculants, as one use of acrylamide mentioned above, have recently found wider applications in waste water treatment, etc., and considerable efforts have been directed to improvement of their quality and performance. In particular, the molecular weight of acrylamide polymers used as flocculants, which is said to contribute directly to the performance of flocculants, has tended to become higher, and high molecular weights of more than 10 million, particularly about 14 million, are currently sought. This is very unique in view of the fact that molecular weights required of acrylamide polymers for other uses or of other polymers are usually less than one million. In addition, since as flocculants, acrylamide polymers are normally used in the form of an aqueous solution, they are required to dissolve rapidly in water without leaving any insoluble portion. Moreover, because of the toxicity of the acrylamide monomer, the content of the unreacted monomer remaining in the polymer is required to be very low, for example, not more than 0.2%. Usually, however, these requirements are difficult to fulfill together with the requirement for increasing the molecular weight, and a great deal of effort has been made to achieve these improvements simultaneously.
The "molecular weights", as used in this application, refer to those determined by the test method shown in Example 1 given hereinbelow. The "water solubility of an acrylamide polymer", as used in this application, refers mainly to that of a dry powder having a water content of not more than 20% by weight, especially about 10% by weight, obtained by drying a polymer prepared usually in an aqueous medium.
Many suggestions have been made about the production of high-molecular-weight acrylamide polymers having good water solubility. They include, for example, the addition of urea-type compounds, various amines, nitrilotriscarboxylic acids, etc., as an insolubilization inhibitor before, during or after the polymerization reaction of acrylamide; the use of a specified polymerization initiator system such as a combination of a cerium salt and acetylacetone or a combination of an oil-soluble azo compound and an amine; and a method in which drying of a hydrogel obtained by the polymerization reaction is effected by jointly using extractive dehydration with a solvent, or the drying is effected in two stages under different conditions.
It is recognized that the solution to the aforesaid problem depends not only upon the method of production of acrylamide polymers, but also greatly upon the quality of acrylamide. For example, Japanese Laid-Open Patent Publication No. 68118/1977 states that the acrolein content of the starting acrylonitrile should be not more than 1.5 ppm, and Japanese Laid-Open Patent Publication No. 138585/1977 states that the 3,3',3"-nitrilotrispropionic acid content in acrylamide should be adjusted to 0.1 ppm or below. Thus, even about 1 ppm of an organic impurity in acrylamide or in the starting acrylonitrile is regarded as being toxic, and a very high degree of purification would be necessary. As is well known, acrylamide is a very reactive compound which undergoes vinyl-type polymerization reaction, carbamoyethylation, a reaction involving transfer of hydrogen of the amide group, etc. Hence, it is likely to induce such reactions during purification to generate new impurities.
From this standpoint, the crystallization method heretofor practiced for purification of acrylamide is an accurate and superior method, but adds greatly to the cost of production. This is because by the catalytic hydration process, acrylamide is inevitably formed in aqueous solution and is marketed as such, while on the other hand, acrylamide polymers are most generally produced by aqueous solution polymerization or oil-in-water emulsion polymerization using acrylamide in the form of an aqueous solution. In view of this, the inclusion of a crystallization step in the process of producing acrylamide monomer is very uneconomical.
Many methods have been proposed for purifying acrylamide obtained by the catalytic hydration process while it is in the form of an aqueous solution. They include, for example, a method which comprises distilling off the unreacted acrylonitrile together with a part of water under weakly basic conditions (Japanese Laid-Open Patent Publication No. 56914/1974), a method which comprises eliminating copper using a specified cation exchange resin (Japanese Laid-Open Patent Publication No. 62929/1975), a method which comprises maintaining the acrylamide solution under basic conditions while blowing an inert gas thereinto (Japanese Laid-Open Patent Publication No. 133318/1974), a method which comprises treating the acrylamide solution with a strongly basic anion exchange resin (Japanese Laid-Open Patent Publication No. 82011/1975), and a method which comprises subjecting the acrylamide solution to air treatment and treatment with a strongly acidic cation exchange resin, followed by treatment with a weakly basic anion exchange resin (Japanese Laid-Open Patent Publication No. 100418/1977).
The present inventors investigated these methods and their combinations in detail, but failed to find a method for purifying acrylamide which is suitable for production of the aforesaid high-molecular-weight acrylamide polymer. When a dry powdery product of a high-molecular-weight acrylamide polymer is produced by using the acrylamide purified by the above methods, its quality, especially its water solubility, is frequently unsatisfactory. Furthermore, when the above acrylamide is stored for a long period of time as an aqueous solution, it is degraded during storage, and polymers produced from the stored acrylamide have reduced solubility in water.